GEOCHEMISTRY OF THE EARLY CARBONIFEROUS WUSU VOLCANIC,MOJIANG COUNTY,YUNNAN, AND ITS TECTONIC SIGNIFICANCE

The Wusu volcanics,west of the Ailaoshan Suture Zone,are located in the Simao Massif.They are composed of basalt,pillow lava,dacite,rhyolite and tuff,about 800m thick,occurring in the early Carboniferous sequence.As compared to N-MORB,the Wusu basalt has a higher content of TiO₂ and REE,and higher Ti/V and Zr/Y ratios characteristic of E-MORB.The basalts are LREE-enriched,with La_N/Yb_N=1.9-2.4,Yb_N=11-23.The rhyolites are higher in REE than the basalts,but with La_N/Yb_N=2.0-2.8.This shows that both of them may have the same origin,i.e. the bi-modal type.Such composition of the Wusu basalt indicates an extension environment rather than an island arc environment.The higher Th/Ta ratios (1.4-4.0) may imply a hibridization with the continental crust during its rise to the surface.